SECT. I. ORGANOGBAPHY AND GLOSSOLOGY. 0.3 



however they may afterwards be curved (fig. 54.). The 

 angle at which they diverge is generally acute, towards 

 the apex of the limb, and their mode of ramification 

 bears a resemblance to the branching of trees. This 

 kind of nervation may be subdivided into four sub- 

 ordinate groups, which are important in regulating 

 the conditions upon which some of the principal forms 

 of leaves depend. 



(a.) Penninerved. Here the midrib is continued to 

 the extremity of the limb, and the primary nerves 

 branch off from it on either side, throughout its whole 

 length (fig. 56.). The breadth of the leaf is chiefly 

 regulated by the size of the an- 56 

 gle at which the nerves quit the 

 midrib, being narrower in pro- 

 portion as this angle is more 

 acute. The contour of the limb 

 is also defined by the proportion 

 which the different nerves bear 

 to each other on quitting differ- 

 ent parts of the midrib. This 

 form of nervation is by far the 

 most usual, and regulates the 

 structure of many compound 

 leaves. In these the main petiole 

 may be likened to the midrib of a 

 simple leaf, with its parenchyma 

 only partially developed round the secondary nerves, so 

 that it becomes split up into separate leaflets. Compound 

 leaves are pinnate, bi-, tri-, &c. pinnate, according 

 to the degree of subdivision to which the branching of 

 the petiole extends. But when the limb of a leaf is 

 merely subdivided, without being completely separated 

 into distinct leaflets, the terms applied to designate the 

 degree of subdivision are " pinnatifid," " bi-, tri-, &c. 

 pinnatifid." In pinnate leaves, the leaflets are fre- 

 quently arranged in pairs, on opposite sides of the 

 petiole., with or without a terminal leaflet. 



